This invention relates to the production of lamp filament grade ammonium paratungstate from tungsten ores and, more particularly, to a purification process which is operable to produce lamp grade ammonium paratungstate from any of a variety of tungsten ores.
Tungsten bearing ores such as wolframite (a tungstate of iron and manganese), scheelite (native calcium tungstate), or ferberite (predominantly iron tungstate) have all been used in various refinement processes in order to produce highly purified lamp grade ammonium paratungstate crystals which have the formula (NH.sub.4).sub.10 W.sub.12 O.sub.41. .times. H.sub.2 O. The purification steps have normally involved forming alkali tungstate solutions to remove impurities and ultimately forming a highly purified ammonium tungstate solution which is then crystallized to the paratungstate form. Many of there prior art processes have been adapted to a batch type operation and very few are operable to refine more than one specific type of ore for which the process is designed.
Typical tungsten refining processes are set forth in U.S. Pat. No. 3,193,347 dated July 6, 1965 to Forward et al. which discloses that scheelite tungsten ore is digested with hydrochloric acid to produce a solid mixture which contains tungstic acid and gangue or waste material. After separation of solids from the solution, such as by filtration, the tungstic acid content is dissolved in sodium hydroxide to produce sodium tungstate which is then separated from the undissolved residue by filtration.
U.S. Pat. No. 1,293,404 dated Feb. 4, 1919 to Giles et al. discloses at page 1 the use of calcium chloride for purifying alkali tungstate solutions. Giles also discloses that any compound of other alkaline earth metals may be used instead of the calcium chloride to effect the purification.
U.S. Pat. No. 1,399,705 dated Dec. 6, 1921 to Ekeley et al. discloses at page 1, line 104, that an oxidizing agent can be added to the sodium tungstate solution and thereafter, magnesium salt such as the sulphate or chloride is added to the solution in a quantity sufficient to combine with phosphorus and arsenic in order to remove same. At page 2, line 15-25 the patentee discloses that the solution is rendered basic by the addition of commercial ammonia and by the further addition of ammonium chloride which precipitates ammonium magnesium phosphate impurity.
U.S. Pat. No. 3,256,057 dated June 14, 1966 to Burwell discloses at column 1, lines 40-50 the removal of molybdenum from scheelite ores by adding sodium sulfide, sodium bisulfide or ammonium sulfide to an alkali tungstate solution in amount more than sufficient to form molybdenum trisulfide of all of the molybdenum present, and adding sufficient mineral acid to provide a pH of at least 3. The solution is then brought to the temperature of 80.degree. C or more and the molybdenum sulfide precipitate is separated from the solution by filtration.
The technique of extraction of tungsten values from an acidic sodium tungstate solution is disclosed in South African Pat. No. 684,892 dated July 17, 1968. This Patent discloses specific extractives which can be any of a variety of amines, a so-called phase conditioner which is used to enhance the efficiency of the extractive, and a diluent such as kerosene. This South African Patent also discloses stripping the tungsten values from the organic solvent with ammonium hydroxide. Thereafter, ammonium paratungstate is produced from an ammonium tungstate solution.